Broadcasting of e-cigarette adverts has proved highly controversial and some health campaigners have questioned whether they indirectly promote smoking, prompting the advertising watchdog to launch an investigation into marketing practices in the sector.

E-cigarette advertising has only been allowed on TV since January 2013.

Overall, 31,136 complaints were made about 18,580 ads last year, according to the latest Advertising Standards Authority (ASA) annual report.

This was slightly down on 2012 when 31,298 complaints were made about 18,990 ads.

Television remained the most complained media channel in 2013, registering 13,179 complaints, ahead of the 9,988 internet-related complaints. Online ads now represent a third of the watchdog's workload, the ASA said.

The third most complained media channel was outdoor advertising which recieved 1,180 complaints.

Across sectors, leisure was the complained about sector, registering 5,119 complaints, ahead of retail with 4,467 compaints, and third placed food and drink with 3,988 complaints.

The VIP ad was the most complained about of last year but the second, with nearly 200 fewer complaints, was a TV and online ad for Unilever-owned Marmite.

The Marmite ad, in the style of a fly-on-the-wall documentary, followed rescue officers visiting people's houses looking for neglected jars of Martmite.

Viewers complained it trivialised the work of child and animal abuse services, although the advertising watchdog decided against banning it.

Likewise, the ASA decided against banning the third most complained about ad, also by Unilever, an animated TV and online ad for Flora Buttery margarine featuring two siblings wrestling. The ad attracted 513 complaints about its "mild sexual" references.

Fifth spot was taken by a YouTube and TV ad for Irn-Bru, which attracted 223 complaints but was not banned. It featured a mother embarrassing her son by wearing a push-up bra and pressing her head against her bosom in front of his friends.

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